Someone who uses agism, usually by an adult who obviously cannot remember his or her childhood becuase if they did they'd think that kids deserve rights.

TV-14 is an agist rating.
Saying that kids under 21 can't look at adult porn is agist.
Saying that kids under 18 can't vote is also agist. Maybe their vote shouldn't have been rated as well as the adults' votes are, but EVERYONE deserves their opinion.

This is what is NOT a form of agism:
Saying that kids under 16 aren't allowed to drive a car.
Saying that kids under 21 can't drink beer.
Saying that someone at a certain age is better than someone at another age (agism is about age rights, not abilities.)